Thrombotic cardiovascular (CV) diseases, including heart attack (MI), stroke and deep venous thrombosis (DVT), are the leading cause of death in developed countries. Using transcriptional profiling of platelets, we identified the S100 calcium-modulated protein family member MRP-14 (S100A9) as an acute MI gene. Case- control validation studies demonstrated that elevated plasma levels of MRP-8/14 complexes predict increased risk of future CV events. Studies using MRP-14-/- mice determined that MRP-8/14 broadly regulates vascular inflammation. However, a key unanswered question is whether MRP-8/14 participates directly in thrombosis. In a recent report (Wang et al. J Clin Invest 2014), we showed that the time to arterial thrombotic occlusion was prolonged markedly in MRP-14-/- mice. We observed that MRP-14 and MRP-8/14 are expressed in and secreted by platelets, and that thrombus formation is reduced in whole blood from MRP-14-/- mice. Infusion of WT platelets or purified MRP-14 into MRP-14-/- mice shortened the carotid artery occlusion time, indicating that platelet-derived MRP-14 directly regulates thrombosis. We next identified CD36 as the platelet membrane receptor for MRP-14. Importantly, while deficiency of MRP-14 is protective of thrombosis, it has no effect on multiple parameters of hemostasis. The central hypotheses of this project are that platelet MRP-14 regulates arterial and venous thrombosis in a CD36-dependent manner, that this interaction can be exploited to develop a safer anti-thrombotic agent (i.e., reduced bleeding risk), and that plasma MRP- 8/14 levels are genetically determined and modified by CV disease activity. We propose 3 specific aims. First, we will characterize the molecular domains responsible for MRP-14:CD36 binding and the downstream signaling that leads to platelet activation. Second, we will investigate the role of MRP-14 in venous thrombosis and in arterial thrombosis in the context of atherosclerosis. Third, major genetic, cellular, and CV disease activity determinants of plasma MRP-8/14 concentration will be explored in collaboration with the Genes and Blood Clotting Study. The MRP-14:CD36 interaction represents a novel target for treating cardiovascular disorders, including heart attack stroke, and DVT. The results of these studies will provide important insights to exploit this interaction to influence thrombosis, but not hemostasis (i.e., reduced bleeding risk).